This application is directed at better understanding how the brain serotonin system functions under normal conditions. Only through such an understanding will we be able to appreciate (and ameliorate) how its dysfunction is manifested in a variety of clinical pathologies, including, depression, anxiety, obsessive-compulsive disorders, and myoclonus. In order to achieve a necessary level of experimental precision, the proposed experiments focus on single cell recordings in behaving animals. Previous studies have characterized the activity of brain serotonergic neurons across the sleep-wake-arousal cycle, and their response to a variety of environmental and physiological challenges. The proposed studies will extend these analyses to an examination of the activity of these neurons during complex mammalian behavior. In particular, pilot experiments indicate that serotonergic neurons in the rostral (pontine) raphe groups are dramatically activated (2-5 fold) during central pattern generator (CPG) mediated rhythmic behaviors, especially those involving oral-buccal responses (chewing, licking, grooming). Reciprocally, the activity of many (all) of the same neurons is strongly inhibited during orienting responses (as indicated by head and eye movements, hippocampal theta activity, PGO waves, and heart rate deceleration). These initial observations will be tested rigorously, and expanded to encompass the caudal (medullary) group of serotonergic neurons. Three sets of studies are proposed, utilizing single unit recordings, multibarrel microiontophoresis, and in vivo brain microdialysis. I) SEROTONERGIC NEURONAL ACTIVITY AND CPG MEDIATED BEHAVIORS. a) Does a definitive positive relationship exist between the activity of pontine serotonergic neurons and rhythmic behaviors, especially those involving the oral-buccal region? b) Do the medullary groups of serotonergic neurons display a relation to rhythmic behaviors which does not include oral-buccal movements, but that does include movements mediated by more caudal aspects of the neuraxis? c) Are CPG-related increases in neuronal activity accompanied by increased release of serotonin preferentially in those brain regions directly related to the behavior? II) RELATION OF SEROTONERGIC NEURONAL ACTIVITY TO ORIENTING RESPONSES. a) Does a definitive inverse relationship exist between the activity of pontine, but not medullary, serotonergic neurons and orienting behavior? b) Is this decrease in neuronal activity related to eye or head movements, or to afferent inputs? III) ROLE OF THE SOMATODENDRITIC AUTORECEPTOR UPON SEROTONERGIC NEURONAL ACTIVITY UNDER PHYSIOLOGICAL CONDITIONS.